* Wash the rice in several changes of cold water and drain then leave it to sit for about 30 minutes.
* Meanwhile combine the vinegar with the sugar and salt in a small pan and bring to a bubble.
* Remove from the heat stir until the sugar has dissolved and leave to go cold.
* Next put the drained rice into a deep saucepan and level it off.
* Add 650ml water cover and bring to a bubble.
* Cook for 2 minutes then reduce the heat to very low and leave for a further 8 minutes without lifting the lid.
* Remove from the heat uncover the pan cover with a tea towel and replace the lid on top.
* Leave to stand for 5 minutes.
* Then using a wooden spatula or spoon gently turn the grains through; cover again with the towel and lid and leave for a further 5 minutes.
* Next turn out onto a deep tray and using a wooden implement again paddle in about 3/4 of the vinegar mixture: taste and add extra if required. While paddling it’s a good idea to fan the rice so that it cools rapidly this will also enhance its glossiness.
* Mix the soy sauce with the mirin and when you’re near to serving slice the tuna into domino like shapes with a sharp knife then bathe them for just 5 minutes (and no longer) in the sweet soy mixture.
* Meanwhile sprinkle the cucumber batons with a few drops of rice vinegar and mix the grated white radish with the chili if using.
* To serve mound the sushi rice into small bowls arrange a little stack of cucumber and tuna on each then add a teaspoon of the hot radish a tuft of spring onion and a scattering of toasted sesame seeds.